Better error message for unconfirmed users when using git

What does this MR do?

Related issue: #231597 (closed)

In 13.2 we automatically unconfirmed wrongfully verified email addresses due to a security bug (gitlab-com/www-gitlab-com#7942 (closed)). Users who didn't check their email might be surprised seeing "blocked" comments when executing git operations. This MR introduces a special case in UserAccessDeniedReason where we give more details about the unconfirmed state and give link to request a new confirmation email.

How to test it:

1. Make sure you have enabled SSH in GDK.
2. Set up ssh key and a project.
3. Make your user "unconfirmed": user.update!(confirmed_at: nil, confirmation_token: 'a' * 32)
4. Pull the project via SSH.
5. You should see the "email is not confirmed" error message.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team